Cast iron is made in various grades that can be categorized by the graphite morphology. In the case of gray iron castings, the flaked graphite structure is predominant. Theses iron grades are not treated with magnesium and the dissolved oxygen content is controlled by the silicon-manganese complex deoxidation equilibrium. This control results in nonmetallic oxide inclusions that are plastic at temperatures present during machining. The deformable nature of these manganese silicate inclusions is in part responsible for the free machining behavior of gray cast iron.
In the case of magnesium treated iron, the oxygen is controlled by the magnesium-oxygen equilibrium, and the resulting non-metallic inclusions are magnesium silicates or magnesium oxide. These inclusions are not plastic at the temperatures attained during any machining process. Therefore, they are not useful in the processes of tool lubrication or chip formation.